Optical reflectometry finds wide application in optical sensing, its principle being: a beam of detection light is sent towards an optical fiber under test (FUT), and the intensity curve of a back scattered light detected via a photodetector is obtained to analyze loss and breaking points in the FUT. Traditional optical time-domain reflectometry uses an optical pulse as the detection light, and therefore its spatial resolution is determined by the pulse width, the narrower the pulse width, the higher the spatial resolution. However, in consideration of limitation on the output power of the optical pulse, the narrower the pulse width, the smaller its energy, which implicates that the detection light is liable of being submerged in noise. [M. K. Barnoski, M. D. Rourke, S. M. Jensen, R. T. Melville, “Optical time domain reflectometer,”Applied Optics, vol. 16, no. 9, pp. 2375-2379, 1977]. Hence, spatial resolution and measurement range in traditional optical time-domain technology are mutually restrictive with each other.
To overcome the bottlenecks in traditional optical time-domain reflectometry, optical frequency domain reflectometry is proposed. Optical frequency domain reflectometry uses continuous linear frequency modulation light as detection light, with a phase difference existing between optical signals reflected from different displacements in the fiber and the continuous linear modulation light to form optical beat frequency signals with various frequency differences. Said signals are transformed to photocurrents via a photoelectric detector and then mapped to frequency domains to obtain reflectometry information of the fiber. Spatial resolution of the frequency domain reflectometry depends only on the sweeping range of the linear modulation frequency, and hence there is no constraint between the spatial resolution and the measurement range. However, the measurement range is limited by the coherent length of the optical source, the maximum measurement range being approximately half of the coherent length, if the detection light were not to be being submerged in noise. [D. Uttam and B. Culshaw, “Precision time domain reflectometry in optical fiber systems using a frequency modulated continuous wave ranging technique,”Journal of Lightwave Technology, vol. 3, no. 5, pp. 971-977, 1985].
In comparison with radar technology, traditional optical time domain reflectometry is similar to pulse radar systems, while optical frequency domain reflectometry is likened in its working mechanism to frequency modulated continuous wave radar. In radar technology, there is a pulse compression technique with no contradictory constraints between the spatial resolution and the measurement range, whose spatial resolution depends only on its sweeping range, and whose measurement range goes farther than the frequency modulated continuous wave radar. [M. A. Richards, Fundamentals of radar signal processing, McGraw-Hill Education, 2005].
Therefore, by applying pulse compression radar techniques in optical time domain reflectometry, the contradictory constraint between spatial resolution and measurement range in traditional optical time domain reflectometry shall be overcome, superb spatial resolution of the pulse compression shall be in full display, and its measurement range shall be longer than that in the optical frequency domain reflectometry.